This invention relates generally to a thermosetting resin composition and, more particularly, to an epoxy resin composition useful for insulation of stator or rotor coils.
In the field of automobiles, there are increasing demands for compact motors such as for use in starters in correspondence to the recent demands for minimizing the space of engine rooms. Since the revolution speed of such a smallsized motor must be increased to, for example, as high as several r.p.m. in order to obtain a desired output torque, the current density of the coil is increased so that the temperature of the rotor becomes as high as about 350.degree. C. during use. As a result, when a conventional vanish is used for fixing and insulating the coils, cracks or blisters are formed in the vanish coat during operation of the motor, so that there are caused insulation failure and loosening of the windings.
An epoxy resin composition has also been used for fixing stator coils or armature coils. One such known epoxy resin composition is composed of 3,4-epoxycyclohexyl-methyl-(3,4-epoxy)cycloheanecarboxylate, methylnadic anhydride and resorcinol (Japanese Unexamined Patent Application No. 57-174314). In use, this composition is mixed with a small amount of triethanolamine and the mixture is applied to a coil to be fixed. The curing is performed at 120.degree. C. for 16 hours and then at 180.degree. C. for 16 hours. Thus, this epoxy composition has a drawback because it requires a long curing time. Another problem of the known epoxy resin composition is that the cured resin is poor in heat-resistance and cannot be used under conditions involving a high temperature of 200.degree. C. or more.